Concrete comprising organic fibres dispersed in a cement matrix, concrete cement matrix and premixes

ABSTRACT

A concrete includes organic fibers, dispersed in a cement matrix. The cement matrix includes elements having predetermined particle sizes. The organic fibers have predetermined lengths and diameters. The behavior of the concrete is improved both with respect to the occurrence of minute cracks and the propagation of large cracks as a result of a synergistic effect between the cement matrix and the organic fibers.

This invention relates to a new fiber concrete allowing to makestructure components and having better properties than those of theprior art components, in particular with respect to the tensile stressbehaviour (bending and direct tensile stress). The fibres being used areorganic fibres.

A structural analysis of concrete has shown that their mechanicalproperties are closely linked to the presence of structural defects.Many types of structural defects can be observed in these concrete mixeswhen they are subject to mechanical loads. They differentiate from eachother with their size.

At the lowest scale, the so-called microporosity defect of the concreteis observed, that means so-called capillary pores, derived fromintergranular spaces initially present in the fresh paste. Their sizelies in the range from 50 nanometers to a few micrometers.

At an intermediary scale, microcrack defects are observed. These aremicrocracks having openings in the range between 1 and 100 micrometers.These are non coalescent, i.e. they do not form a continuous paththrough the structure. These are essentially due to the heterogeneouscharacter of concrete, the granulates having mechanical and physicalproperties different from those of the binder/cement. They occur uponmechanical loading. This defect type is mainly responsible for the poormechanical tensile stress properties of concrete and its breakablecharacter.

At the upper scale, macrocrack defects are observed. The crack openingvaries from 100 μm to 1 mm. These cracks are coalescent.

Millimetric size major defects may also be observed which are due to awrong concrete preparation (occluded air, filling defects).

Solutions have been proposed either to reduce the presence of thesevarious defects or to attenuate their effects.

Thus, it has been possible to partially control microporosity byreducing the water/cement weight ratio and using fluidizing agents. Theuse of fine fillers, in particular with a pozzolanic reaction, has alsomade it possible to reduce the micropore size.

As far as microcracking is concerned, it has been strongly reduced:

by improving the concrete homogeneity, for example, by reducing thegranulate size to 800 μm,

by improving the material compactness (granular optimization andoptional pressing before and during setting),

thermal treatments after setting.

As to microcracking, it has been controlled by the use of metal fibres.

WO-A-95/01316 can be mentioned as prior art document. It relates tocontrolling the size ratio between the metal fibres and the granularelements (sand, granulates). This improved fibre concrete comprisescement, granular elements, fine elements with pozzolanic reaction andmetal fibres. The granular elements must have a maximum grain size D of800 micrometers at the most, the fibres must have an individual length Iin the range between 4 mm and 20 mm and the ratio R between the averagelength L of the fibres and D should at least equal 10.

The resulting concrete shows a flexural ductile behaviour or pseudo coldworking.

Concrete or mortar formulations comprising organic fibres have also beensuggested for various purposes, optionally conjugated with metal fibres,as disclosed, for example, in the publication “Fibre reinforcedcementitious composites” by A. BENTUR, S. MINDESS (Elsevier AppliedScience, 1990).

The state of the art shows that the man skilled in the art who aims atformulating a fibre concrete faces multiple possible choices ofmaterials and proportions, as well as regards the concrete cement matrixthan the fibres, so that the problem still remains that a concrete is tobe formulated having improved properties compared to existing concretemixes and the cost of which is not to be redhibitory for its efficientuse in the building industry and public works.

An answer to the aimed properties is to be found at the level of the useof organic fibres instead of metal fibres: increase in ductility, inparticular tensile stress, reduction of corrosive effects, weightreduction of fibre concrete structures. A less important attenuation ofthe radioelectric signals can also be mentioned.

An interesting effect provided by the presence of polymer typereinforcing fibres is an improved fire behaviour of the fibre concretemixes.

Another further solution is to be found at the level of the eliminationof the above-mentioned defects, more particularly microcracks, becauseit has been observed that the implementations described in the prior artare mainly designed to avoid the development of macrocracks and not ofmicrocracks: the microcracks are not stabilized and develop understress.

The object of the present invention is a concrete mix comprisingreinforcing organic fibres and having improved properties compared tothe prior art concrete mixes, more particularly in tensile stress(flexion and direct tensile stress).

Another aim of the present invention is to provide a concrete mix thecold working of which is improved beyond the first damage by controllingthe macrocrack propagation. The invention aims thus at increasing thefield of use of concrete beyond the first damage by providing a ductilebehaviour to concrete.

FIG. 1 of the accompanying drawings is a typical direct tensile stresscurve of a concrete mix with a ductile nature according to the priorart.

In the case of a break being not of the breakable type (breakable meanshere that break is sudden, not progressive), both the engineer designinga structure and the engineer who calculates it or must check its safety,needs to have access to the behaviour law of the material or to afeature that shows it. The material ductility only corresponds to thenon elastic strain occurring, in direct tensile stress, before theconstraint peak A.

In order to illustrate the advantage of ductility, one can merelyimagine the behaviour of a tie-rod (a strut for example built-in at itsupper end), subjected to an increasing tensile load (weights being addedto the lower end). As soon as this load has reached the peak value, abreak occurs and is complete (in the direct tensile stress test, inparticular, the post-peak portion can only be seen if the test iscarried out upon stress).

The ductility of a non elastic material is characterized by the wholestress-strain curve in a simple tensile stress, but considered only upto the peak. It can also be defined as being the ratio of the breakingstress ε_(A) to the elastic stress ε_(él)=ε_(B)·(σ_(A)/σ_(B))corresponding to the breaking stress (provided σ_(A) is higher than(σ_(B)); this ratio is equal to that of the elastic modulus (OB slope)divided by the secant modulus at break (peak stress divided by peakstrain or OA slope).

The ductility may be described by means of a ductility coefficient δ:$\delta = \frac{\varepsilon_{A} \cdot \sigma_{B}}{\varepsilon_{B} \cdot \sigma_{A}}$

where ε_(A)=peak strain, and$\varepsilon_{e1} = {\varepsilon_{B} \cdot \frac{\sigma_{A}}{\sigma_{B}}}$

with ε_(el)=strain that would be obtained under peak stress byelastically extrapolating the resulting strain under the running stress.

This definition is perfectly in line with the physical behaviourobserved on a test specimen (multicracking): upon the first cracking,the so-called first crack peak B (which is only a local or partialmaximum), is locally reached followed by an opening that can be read onFIG. 1 between first peak B and the point C where the curve goes beyondthe value of this peak; at this time, the first crack is stabilizedbecause stress again increases in the whole stressed volume until theoccurrence of a second crack, etc. This behaviour is strong, as it canonly be more stable in higher size volumes.

Another aim of the present invention is to increase the stress levelwhere the first concrete damage occurs (i.e. microcracks) and thereby toextend the field of use for the concrete (elastic linear behaviour).

Still another aim of the invention is to improve, by a synergisticeffect between the cement matrix and the organic fibres, the behaviourof the concrete both with respect to the occurrence of microcracks andthe propagation of macrocracks.

The aims of the invention have been found to be reached with a concretethat combines a cement matrix with determined features and organicfibres also with determined features.

More precisely, generally speaking, the invention aims a concretecomprising a hardened cement matrix in which organic fibres aredispersed, obtained by blending with water a composition containingbesides organic fibres:

(a) cement,

(b) granular elements with a maximum grain size D of 2 mm at the most,preferably 1 mm at the most,

(c) fine elements with a pozzolanic reaction having an elementaryparticle size of 20 μm at the most, preferably 1 μm at the most,

(d) at least a dispersing agent, and satisfying the followingconditions:

(e) the weight percentage of water E to the added weight of cement (a)and elements (c) is in the range between 8% and 25%,

(f) the fibres have an individual length I of at least 2 mm and a I/φratio, Φ being the fibre diameter, of at least 20,

(g) the ratio R between the average fibre length L and the maximum grainsize D of the granular elements is at least 5,

(h) the fibre amount is such that their volume represents 8% at the mostof the concrete volume after setting.

Thus, with a new design of the granular skeleton and its relationshipwith the reinforcing fibres, this solution solves the problemencountered with this compromise between mechanical properties andrheology.

The concrete properties according to the invention are not substantiallymodified, if within the matrix, granular elements (b) are also used,with a grain size exceeding 2 mm, but in a proportion not exceeding 25%of the volume of all the components (a)+(b)+(c).

The presence of this granular class in such a proportion may beconsidered as a filler that does not take part in the mechanicalperformances of the material in so far as:

the grain size D50 of all the components (a), (b) and (c) is 200 μm atthe most, preferably 150 μm at the most, and

the ratio R between the average fibre length L and the grain size D 75of all the components (a), (b) and (c) is at least 5, preferably atleast 10.

By grain size D75 and D50, it is to be understood the sieve sizes thepassing through part respectively represents 75% and 50% of the totalgrain volume.

The invention therefore also relates to a concrete comprising a hardenedcement matrix in which organic fibres are dispersed, obtained byblending with water a composition containing besides organic fibres:

(a) cement,

(b) granular elements,

(c) elements with a pozzolanic reaction having an elementary particlesize of 1 μm at the most, preferably 0.5 μm at the most,

(d) at least a dispersing agent, and satisfying the followingconditions:

(1) the weight percentage of water E to the added weight C of cement (a)and elements (c) is in the range between 8% and 24%,

(2) the fibres have an individual length I of at least 2 mm and a I/Φratio, Φ being the fibre diameter, of at least 20,

(3) the ratio R between the average fibre length L and the grain sizeD75 of all the granular elements (a), (b) and (c) is at least 5,preferably at least 10,

(4) the fibre amount is such that their volume represents 8% at the mostof the concrete volume after setting,

(5) the whole elements (a), (b) and (c) have a grain size D75 of 2 mm atthe most, preferably 1 mm at the most, and a grain size D50 of 150 μm atthe most, preferably 100 μm at the most.

The conditions (3) and (5) apply to all solid elements (a), (b) and (c)together, without fibres, and not for each element taken individually.

In an alternative, a portion of the organic fibres is substituted for bymetal fibres: a “hybrid” composite is thereby obtained the mechanicalbehaviour of which may be adapted depending upon the requiredperformances (elastic and cold working portion/post-peak portion).

The presence of organic fibres makes it possible to modify the concretefire behaviour as previously defined.

In fact, the melting nature of said fibres makes it possible to developpathways through which steam or water under pressure can escape upon astrong increase of temperature.

The organic fibres may be i.a. selected amongst polyvinyl alcohol fibres(APV), polyacrylonitrile fibres (PAN), polyethylene fibres (PE), highdensity polyethylene fibres (PEHD), polypropylene fibres (PP), homo- orcopolymers, polyamide or polyimide fibres, aramid fibres or carbonfibres as well. Mixtures of these fibres can also be used. Thereinforcing fibres used according to the invention may be selectedamongst the various available fibres on the market and being classifiedin one of the three following categories: high modulus non reactivefibres, low modulus non reactive fibres and reactive fibres. Thefollowing illustrative examples relate, amongst others, to the nonreactive PEHD fibres, the modulus of which is higher than that of theconcrete matrix, the non reactive polyamide fibres (PA), the modulus ofwhich is lower than that of the concrete matrix and the APV fibresreacting with the concrete matrix.

The “hybrid” reinforcing elements may be produced by combining fibres ofvarious natures and/or lengths. The following illustrative examples moreparticularly relate to short APV organic fibres (6 mm) and long metalfibres (13 mm) and show that a considerable reinforcing synergisticeffect is then obtained. Other examples of similar combinations are thefollowing:

APV or PEHD short fibres (6 mm) and APV long fibres (20 mm), short steelcords (5 mm) and APV long fibres (20 mm).

These organic fibres may have the form of an object made either as amonostrand or multistrands, the object diameter being in the range from10 μm to 800 μm. The organic fibres may also be used in the form ofwoven or non woven structures or of a hybrid strand (filament blend).

The individual length of the organic fibres is preferably in the rangebetween 5 mm and 40 mm.

The fibre amount is such that their volume is lower than 8% andpreferably lower than 5% of the concrete volume after setting.

The I/φ ratio, φ being the fibre diameter, is at least 20 and preferably500 at the most.

The tests have shown that even a fibre amount leading to a volume as lowas 1% could be efficient, taking the matrix formulation into account,but that this value should not be considered as a limit value.

In fact, the useful dosages strongly depend upon the fibre geometry,their chemical nature and their intrinsic mechanical properties (elasticmodulus, flowing threshold, mechanical strength).

Using fibre blends having different features allows to adapt theconcrete properties with respect to the desired features.

Advantageously, the average adherence stress of the fibres in the cementmatrix must be at least 2 MPa, preferably at least 5 MPa, depending onthe fibre nature.

This stress is determined by an extraction trial for a monofibre beingembedded within a concrete block, as described hereafter.

The fibre/matrix adherence level may be controlled using several methodswhich can be made individually or simultaneously.

The fibre adherence in the cement matrix can be obtained by reactivitybetween the fibre and the cement matrix, which can be enhanced withthermal treatments carried out on concrete (cure) or with fibre surfacetreatments.

According to a second method, the fibre adherence stress in the cementmatrix can be obtained by including into the composition at least one ofthe following compounds: silica compounds comprising essentially silica,precipitated calcium carbonate, aqueous solution of polyvinyl alcohol,phosphates, latexes, a surfactant (defoaming agent, wetting agent or thelike) or a blend of said compounds.

By silica compounds comprising essentially silica, it is meant synthesisproducts selected amongst precipitated silicas, silica sols,pyrogenation silicas (aerosil type), silico-aluminates, for exampleTixosil 28 marketed by RHODIA Chimie, or the products obtained throughetching natural products of clay type: smectites, magnesium silicates,sepiolites, montmorillonites.

Preferably, at least one precipitated silica is used.

By precipitated silica, it is meant a silica obtained by precipitationfrom the reaction of an alkaline metal silicate with an acid, usuallyinorganic, at a suitable pH of the precipitation medium, particularly abasic, neutral or little acidic pH.

Usually, the precipitated silica amount being introduced is comprised inthe range from 0.1% to 5% in dry weight with respect to the concretetotal composition. Beyond 5%, rheology problems are usually observed inpreparing concrete.

The precipitated silica is preferably introduced into the composition asan aqueous suspension. It can more particularly be an aqueous silicasuspension having:

a dry matter content in the range between 10% and 40% by weight,

a viscosity lower than 4.10⁻² Pa·s for a 50s⁻¹ shearing,

a silica amount contained in the supernatant of said suspension at 7500rpm for 30 minutes higher than 50% of the silica weight contained in thesuspension.

This suspension is more particularly described in Patent applicationWO-A-96/01787. The Rhoximat 60 SL silica suspension marketed by RHODIAChimie is particularly suitable for this concrete type.

Advantageously, the concrete matrix also comprises components able toimprove the matrix tenacity, which are selected amongst needle- orplatelet-shaped elements the average size of which is 1 mm at the mostand being provided in a volume proportion between 2.5% and 35% of theadded volume of the granular (b) and pozzolanic (c) elements. The matrixtenacity is preferably at least 15 J/m², advantageously at least 20J/m².

By “cement matrix”, it is meant the hardened cement composition, withoutfibres.

The granular elements are essentially fine sands or blends of finesands, either sieved or crushed, that can advantageously comprise silicasands, particularly quartz flour.

The maximum size D of these elements is preferably 1 mm or 500 μm at themost.

These granular elements are generally provided in the range of 20% to60% in weight of the cement matrix, preferably 25% to 50% in weight ofsaid matrix.

The ratio R of the average length L of the fibres to the maximum grainsize D of the granular elements is at least 5, particularly when thegranular elements have a maximum grain size of 1 mm.

The cement of the composition according to the invention isadvantageously a Portland cement such as Portland CPA PMES, HP, HPR, CEMI PMES, 52.5 or 52.5R or HTS (high silica content) cements.

The fine elements with a pozzolanic reaction have an elementary particlesize of at least 0.1 μm, and of at the most 20 μm, preferably at themost 0.5 μm. They can be selected amongst silicas, such as fly ashes,blast furnace slags, clay derivatives such as china clay. Silica can bea silica smoke from the zirconium industry instead of a silica smokefrom the silicon industry.

The water/cement weight percentage of the composition according to theinvention may vary when cement substitutes are being used, moreparticularly pozzolanic reaction elements. The water rate is definedwith the weight ratio of the water amount E to the added weight of thecement with the pozzolanic reaction elements: it varies between about 8%and 25% or between 13% and 25%.

The composition according to the invention also comprises a dispersingagent. This dispersing agent is generally a fluidizing agent. Thefluidizing agent can be selected amongst lignosulfonates, casein,polynaphthalenes, in particular alkaline metalpolynaphthalenesulfonates, formaldehyde derivatives, alkaline metalpolyacrylates, alkaline metal polycarboxylates and grafted ethylenepolyoxides. In general, the composition according to the inventioncomprises between 0.5 and 2.5 parts in weight of fluidizing agent for100 parts in weight of cement.

Other additives can be added to the composition according to theinvention, for example, a defoaming agent. Examples can include asdefoaming agents particularly polydimethylsiloxane.

Amongst these agent types, it is worth mentioning in particularsilicones in the form of a solution, a solid and preferably in the formof a resin, an oil or an emulsion, preferably in water. Moreparticularly suitable are silicones essentially comprising patterns M(RSiO_(0.5)) and D (R₂SiO). In these formulae, the R radicals, identicalor different, are more particularly selected amongst hydrogen and alkylgroups comprising 1 to 8 carbon atoms, the methyl group being preferred.The number of patterns is preferably in the range from 30 to 120.

The amount of such an agent in the composition is generally at the most5 parts in weight for 100 parts cement.

All the particle sizes are measured using MET (transmission electronicmicroscopy) or MEB (scanning electronic microscopy).

The concrete is prepared using any well known method by the man skilledin the art, amongst which mixing the solid components and water, shaping(moulding, casting, injection, pumping, extrusion, calendering), andthen hardening.

The resulting concrete may be subjected to a cure for a period of timerequired in order to obtain the desired mechanical features at atemperature from ambient temperature to 100° C., preferably a curebetween 60° C. and 100° C. The cure time may range between 6 hours and 4days with an optimal time being in the order of 2 days, the curestarting after the setting completion of the blend and at least one dayafter the setting start.

The cure is performed in dry or wet conditions or by cycles alternatingboth environments, for instance, a 24 hour cure in humid environmentfollowed by a 24 hour cure in dry environment.

This cure is performed on concrete mixes that have completed theirsetting, preferably at least one day old, and more preferably at least 7day old approximately.

The addition of quartz powder is particularly useful when the concreteis being cured at a high temperature.

The resulting concrete mixes according to the invention usually exhibita direct tensile stress strength Rt of at least 6 MPa, with a behaviourhaving possibly some ductility.

They can also exhibit a 4 point bending strength Rf of at least 20 MPa,a compression strength Rc of at least 140 MPa and a break energy Wf ofat least 2000 J/m².

The tenacity of the cement matrix is obtained particularly by adding tothe cement composition reinforcing agents with an anisotropic shape andan average size of 1 mm at the most, preferably 500 μm at the most.

In general, the reinforcing agents of the composition according to theinvention are present with a needle or a platelet shape.

By micro-reinforcing agent “size”, it is meant the average size of theirlargest dimension (more particularly, the length for the needle shapes).

These agents may be natural or synthesis products.

The needle-shaped reinforcing agents may be selected amongstwollastonite fibres, bauxite fibres, mullite fibres, potassium titanatefibres, silicon carbide fibres, phosphate fibres, for example calciumphosphate fibres, more particularly hydroxyapatite (HAP), cellulose (orits derivatives) fibres, carbon fibres, calcium carbonate fibres,(alkali-resistant) glass fibres. Short fibres (length of at the most 2mm, preferably of at the most 1 mm) of polyvinyl alcohol,polyacrylonitrile, high density polyethylene, polyamide, aramid orpolypropylene may also be used. Materials such as steel wool are alsoincluded in the definition of the reinforcing agents according to theinvention.

The reinforcing agents as platelets may be selected amongst micaplatelets, talc platelets, composite silicate platelets (clays),vermiculite platelets, alumina platelets.

It is possible to use a blend of these various forms or types ofmicro-reinforcing agents in the concrete composition according to theinvention.

These reinforcing agents may exhibit on the surface a polymer organiccoating obtained from at least one of the following components:polyvinyl alcohol, silanes, siliconates, siloxane resins orpolyorganosiloxanes or reaction products between (i) at least onecarboxylic acid containing 3 to 22 carbon atoms, (ii) at least onepolyfunctional aromatic or aliphatic amine or a substituted amine,containing 2 to 25 carbon atoms and (iii) a cross-linking agent which isa hydrosoluble metal complex, containing at least a metal selectedamongst zinc, aluminium, titanium, copper, chromium, iron, zirconium andlead.

The coating thickness may vary between 0.01 μm and 10 μm, preferablybetween 0.1 μm and 1 μm.

The latexes may be selected amongst styrene-butadiene latexes, acryliclatexes, styrene-acrylic latexes, methacrylic latexes, carboxylated andphosphonated latexes. The latexes having calcium complexing functionsare preferred.

The polymer organic coating can be obtained by treating in a fluid bedor using a mixer of FORBERG type the reinforcing agents in the presenceof one of the above-defined compounds.

Preferred compounds include H240 polyorganosiloxane, Rhodorsil 878, 865and 1830 PX siloxane resins, 403/60/WS and WB LS 14 Manalox, allmarketed by RHODIA Chimie, potassium siliconates.

Such a treatment is particularly recommended for reinforcing agents thatare natural products.

The concrete can be either pre-stressed in pre-tension with adherentwire or adherent strand or pre-stressed in post-tension either withgreased sheathed monostrands or with a sheathed cable or bar, the cablebeing made either of a wire assembly or of strands.

The pre-stress, either under pre-tension or post-tension form, isparticularly well suited to concrete products according to theinvention.

The metal pre-stress cables always have very high tensile stressstrengths, wrongly used, as the fragility of the matrix containing themdoes not allow to optimise the dimensions of the concrete structuralelements.

An improvement has already been obtained using high performance concretemixes; in the case of the concrete according to the invention, thematerial is homogeneously reinforced by organic or hybrid fibres, whichallows it to reach high mechanical performances, simultaneously withsome ductility. The pre-stress of this material by using cables orstrands, whatever its mode is, is then nearly completely used, socreating very resistant tensile and flexural pre-stressed concreteelements and therefore optimized.

The volume reduction obtained as a result of this increase in themechanical strengths can generate very light prefabricated elements. Asa result, the possibility is thus offered to have concrete elements of alarge span being easily conveyed due to their light weight; this isparticularly well suited for building large works of art wherepre-stress in post-tension is widely used. The solution then offers forthis work type particularly favorable assembly gains and yard time.

Moreover, the thermal treatment significantly reduces retraction aftercure, limiting thereby the pre-stressed losses in the time.

This property is particularly desired and all the above-mentionedadvantages, associated to the very low permeability of the product,quite favorable for the durability and maintenance of works of art inthe time, make it possible that this material can advantageouslysubstitute for steel works.

The invention also relates to a cement matrix adapted for obtaining andimplementing the above-defined concrete.

Finally, the invention relates to pre-blends containing all or part ofthe components necessary for the preparation of concrete and the matrixas defined here-above.

The following examples illustrate the invention, but without limitingits scope.

SAMPLE PREPARATION 1) Raw Materials

Portland cement: high silica content, HTS type (LAFARGE France)

Sand: BE31 quartz sand (SIFRACO, France)

Quartz flour: C400 grade, 50% of the grains being less than 10 μm(SIFRACO France)

Silica smoke: glass microsilica obtained from the manufacture ofzirconium (SEPR France)

Adjuvant: X 404 (MAPEI Italy) or OPTIMA 100 (CHRYSO, France) liquidsuperplasticizer

Fibres: the organic fibres are APV (KURARAY RM182, RF1500 and RF 4000,UNITIKA 1800), PEHD (DSM—Dyneema) or PA (FILTEC PAK 50) fibres. They arepresent as monostrands with a diameter ranging from 50 μm to 600 μm anda length from 5 to 32 mm. The dosages being used range from 1% to 5% involume (with respect to the total volume)

Needle-shaped reinforcing agent wollastonite (CaSiO₃) NYAD G grade (NYCOUSA)

Platelet-shaped reinforcing agent: mica (muscovite) MG 160 grade(KAOLINS D'ARVOR, France).

2) Manufacture Mode

The components are mixed in the following order:

blending matrix pulverulent components and additional components,

introducing water and part of the adjuvants,

mixing,

introducing the remaining fraction of fluidizing agents,

mixing,

introducing the reinforcing fibres,

mixing.

The mixing duration is strongly dependent upon the mixer type being used(EIRICH or HOBART).

The outgassing can be made easier by mixing at reduced speed at the endof the process.

The moulds are then filled and vibrated according to the usualprocedures.

3) Cure

Maturing at 20° C. The test specimens are released 48 hours aftercasting. They are then subjected to a treatment consisting in storingthem under water at approximately 20° C. for at least 14 days. The testspecimens are machined (depending on the trial to be performed) 26 to 28days after casting and the trial is performed in the following days.

Thermal treatment at 90° C. The test specimens are released 48 hoursafter casting. They are then subjected to a thermal treatment consistingin storing them in an oven at 90° C. for 24 hours in wet air (>90° C.RH), followed by 24 hours in dry air. The machining is performed 6 daysafter casting and the trial is carried out the following days (7 daysminimum after casting).

MEASURING METHODS

Direct Tensile Stress Behaviour: Rt

This is the value obtained in direct tensile stress on dumbbell-shapedtest specimens machined from 70×70×280 mm prisms in order to have auseful section being 70×50 mm² by 50 mm high. The test specimens,carefully aligned, are rigidly mounted on the testing bank (UTS) with asingle freedom degree (no link with knee cardan type articulation).${Rt} = \frac{F\quad \max}{70 \times 50}$

where Fmax represents the maximum strength in N (peak) for a breakingoccurring in the 70×50 central section.

Ductility coefficient: δ

The ductility coefficient δ is defined by the relationship:$\delta \frac{\varepsilon_{A} \cdot \sigma_{B}}{\varepsilon_{B} \cdot \sigma_{A}}$

if σ_(A)≧σ_(B)

where ε_(A) is the deformation at peak, and$\varepsilon_{e1} = {\varepsilon_{B} \cdot \frac{\sigma_{A}}{\sigma_{B}}}$

is the strain that would be obtained under the peak stress byelastically extrapolating the strain obtained under the running stress.

Bending Behaviour: Rf

i) 4 point bending

Rf is the value obtained in 4 point bending (distance between axles:70×210) on 70×70×280 mm prismatic test specimens mounted on knee-shapedbearings.${Rf} = \frac{3F\quad {\max \left( {I - I^{\prime}} \right)}}{2{dw}^{2}}$

where Fmax represents the maximal strength in N (strength at peak),I=210 mm and I′=⅓ and d=w=70 mm.

ii) 3 point bending

The value obtained in 3 point bending Rf (distance between axles: 200)is obtained on 40×40×250 mm prismatic test specimens mounted onknee-shaped bearings. ${Rf} = \frac{3{FmwI}}{2{dw}^{2}}$

where Fmax represents the maximal strength in N (strength at peak),I=200 mm and d=w=40 mm.

Compressive Behaviour: Rc

Rc is the value obtained in direct compression on a rectifiedcylindrical sample (diameter 70 mm/height 140 mm).${Rc} = \frac{4F}{\pi \quad d^{2}}$

where F represents the breaking strength in N and d the sample diameter(70 mm).

Tenacity: Kc, Gc

The tenacity is expressed either in terms of stress (critical stressintensity factor: Kc) or in terms of energy (energy critical rate: Gc),using the formalism of the Break Linear Mechanics.

The trials are performed in 3 point bending from notched 40×40×250 or70×70×280 mm prisms, i.e. SENB geometry samples (ASTM-E 399-83 Method).A V-profiled notch is dry made on these prisms, using a milling machineprovided with a diamond disc. The relative depth a/w of the notch is 0.4(a: notch depth, w: sample height).

The stress intensity critical factor Kc is obtained from the breakingload F and the crack length a at the instability point (servo movingtest, at 10⁻² mm/s, on SCHENCK universal test machine:${Kc} = {\frac{3{FI}}{2{dw}^{2}}\sqrt{aY}}$

where:

I represents the distance between axles between supporting points(bending bench):=210 mm,

d and w are respectively the depth and the height of the sample,

a is the length notch during breaking,

Y is a shape parameter depending on the crack length (α=a/w). In 3 pointbending, the Y parameter according to SRAWLEY J. E (International J. ofFracture (1976), vol. 12, pp. 475-476) is preferably used:$Y = \frac{1.99 - {{\alpha \left( {1 - \alpha} \right)}\left( {2.15 - {3.93\alpha} + {2.7\alpha^{2}}} \right)}}{\left( {1 + {2\alpha}} \right)\left( {1 - \alpha} \right)^{3/2}}$

 Gc can be obtained from the strength-shift curves on the condition thatthe contributions due to interference strains are extracted and that thedissipated energy is reported to the ligament section: (w−a)×d.

In planar strain, there is a simple relationship between Kc and Gc:${Gc} = \frac{{Kc}^{2}\left( {1 - v^{2}} \right)}{E}$

where:

E is the elastic modulus,

v represents the Poisson's coefficient.

E is experimentally obtained by vibrating a prismatic sample bearing ontwo supports from the fundamental frequency determination (GRINDO-SONICmethod).

Breaking Energy: Wf

Wf is the value obtained in determining the total area under theforce-sag curve, upon a 4 point bending trial on 70×70×280 mm prisms.The measured sag is corrected in order to determine the real sampleshift. ${Wf} = \frac{\int{F\quad \delta \quad c}}{dw}$

where F is the force being applied, δc is the real shift (correctedsag), d×w the sample section.

Adherence

As to the organic fibre adherence in the cement matrix, the stress isdetermined by an extraction test for a monofibre embedded in a concreteblock.

The wires are embedded in concrete blocks the dimensions of which are4×4×4 cm. The composition being used is the same as that used for thetest specimens in the mechanical trial (bending, compression andtension): the water/cement ratio is set at 0.25.

The wires embedded in a length of 10 mm are extracted by tension using auniversal test machine (SCHENCK) with a speed of 0.1 mm/min.

The stress being exerted is measured by means of an adapted force sensorand the wire shift (with respect to the sample) by means of anextensometry sensor.

The average adherence stress is evaluated from the following simplifiedformula: $\tau_{d} = \frac{F\quad \max}{{\pi\varphi}\quad I_{e}}$

where Fmax is the maximum force being measured, φ the wire diameter andI_(e) the embedding length.

EXAMPLES

Fibre concrete mixes are produced using the fibres defined in thefollowing tables I to VI, the compositions of these concrete mixes beingdefined in the following tables II to VI. These compositions are basedin weight.

The performances of said concrete mixes are indicated in the followingtables II to V, as well as on FIGS. 2 to 14.

In the figures:

FIG. 1 is a typical direct tensile stress curve of a concrete mix with aductile nature.

FIG. 2 is a graph obtained by 4 point bending trials with in ordinate,the stress values (MPa) and in abscissa, the values of the sag (mm) forconcrete samples, with a E/C ratio=0.2 and a maturation at 20° C. (28days): comparison of the steel fibres (Steel Cord) and the organicfibres (APV).

FIG. 3 is a graph obtained by 4 point bending trials with in ordinate,the stress values (MPa) and in abscissa, the values of the sag (mm) forconcrete samples, with a E/C ratio=0.2 and a thermal treatment at 90°C.: comparison of the steel fibres (Steel Cord) and the organic fibres(APV).

FIG. 4 shows a graph obtained by direct tensile stress tests with, inordinate, the stress values (MPa) and in abscissa, the elongation values(mm) for concrete samples, with a E/C ratio=0.2 and a maturation at 20°C. (28 days): organic fibres (APV).

FIG. 5 shows a graph obtained by direct tensile stress tests with, inordinate, the stress values (MPa) and in abscissa, the elongation values(mm) for concrete samples, with a E/C ratio=0.2 and a thermal treatmentat 90° C.: organic fibres (APV).

FIG. 6 shows a graph obtained by direct tensile stress tests with, inordinate, the stress values (MPa) and in abscissa, the elongation values(mm) for wollastonite-containing concrete samples, with a E/C ratio=0.24and a maturation at 20° C. (28 days): organic fibres (APV).

The ductility criterion 6 has varied from 3 to 5 approximately.

FIG. 7 is a graph obtained by 3 point bending trials with, in ordinate,the force (N) and in abscissa, the values of the shift (mm) for concretesamples, with a E/C ratio=0.25 and a thermal treatment at 90° C.: APV RF1500 fibres.

FIG. 8 is a graph obtained by 3 point bending trials with, in ordinate,the force (N) and in abscissa, the values of the shift (mm) for concretesamples, with a E/C ratio=0.25 and a thermal treatment at 90° C.: APV RF1500 fibres of different length (10 mm to 30 mm).

FIG. 9 is a graph obtained by 3 point bending trials with, in ordinate,the force (N) and in abscissa, the values of the shift (mm) for concretesamples, with a E/C ratio=0.25 and a thermal treatment at 90° C.: PEHDfibres.

FIG. 10 is a graph obtained by 3 point bending trials showing the effectof a blend of APV organic fibres (2% vol. of RF 1500 and 2% vol. of RF4000) in a concrete matrix with a E/C ratio=0.25 and a thermal treatmentat 90° C. for 48 hours.

FIG. 11 is a graph representing the stress/shift curves obtained in 3point bending trials with compositions 18 and 19 with the PEHD fibres intable V.

FIG. 12 is a graph similar to that of FIG. 11 obtained with compositions20 and 21 with the PA fibres in table V.

FIG. 13 is a graph similar to that of FIG. 11 and 12 obtained withcompositions 22, 23 and 24 with APV fibres and 25 with APV/steel hybridsin table V.

FIG. 14 is a graph similar to that of FIG. 11 to 13 for comparing thebehaviours of fibres of different types according to compositions 18(PEHD), 20 (PA) and 23 (APV) in table V.

FIG. 15 is a graph illustrating the results obtained in the monofibrepull-out tests with various fibre types, with in ordinate, the forcebeing applied and in abscissa, the shift, with the matrix of thecomposition indicated in table VI.

The results obtained in 3 point bending in examples 18 to 25 correspondto trials performed using a 120 mm distance between axles on 40×40×160mm prismatic test specimens.

The reinforcing effect generated by fibres dispersed in a concretematrix is clearly enhanced through (4 point) bending trials: FIGS. 2 and3. The polyvinyl alcohol fibres (APV) incorporated at a rate of 4% vol.lead to a behaviour similar to that obtained with steel cord dispersion(2% vol.). The thermal treatment (90° C.) develops some reactivitybetween the APV fibres and the concrete matrix: hence, a high peakstress observed in (4 point) bending.

In direct tension stress (FIGS. 4 and 5), an important cold workingeffect (ductility) is observed in the presence of 4% vol. polyvinylalcohol fibres (APV): an important multicracking is observed on thetensile stress test specimens. This phenomenon is not observed in thecase of steel cords because of their high rigidity and the averageadherence of such fibres in the concrete matrix. The peak value issubstantially improved in the case of a 90° C. thermal treatment.

Similar behaviours are observed in the case of APV fibres dispersed in awollastonite-containing matrix (FIG. 6), with the first damage stressincrease.

In the presence of organic fibres, the concrete mixes tested in 3 pointbending show a high ductility: high cold working until the peak stress,post-peak dissipated energy. This is observed both for APV polyvinylalcohol fibres (FIG. 7) and for PEHD high density polyethylene fibres(FIG. 9).

The fibre length must be optimised: compromise between rheology andmechanical properties. For example, for APV fibres having approximatelya 400 μm diameter, it has been observed in 3 point bending that there isa transition regarding the fibre length towards 18 mm approximately(FIG. 8). This gives a elongation factor in the order of 50.

The advantage of a fibre blend is illustrated in FIG. 10. It has beenobserved in the one hand that APV fibres of average dimensions (KURARAY1500: 4% vol.) lead to a bending strength gain (peak stress) of theconcrete: on the other hand, higher dimension APV fibres (KURA Y 4000:4% vol.) induce an important energy dissipation in bending (post-peakportion), but to the detriment of the resistance (peak stress). Acombination of the two fibre types surprisingly allows to obtain areinforced concrete having (for instance in bending) both an improvementof the bending resistance (peak stress) and of the dissipated energy(ductility): (FIG. 10).

Effects for this purpose can be developed by an hybrid reinforcement:combination of organic fibres and metal fibres.

The invention is not limited to the embodiments that have beendescribed.

It also encompasses the concrete mixes that, besides the claimedcompositions or equivalent compositions, would comprise additionalconstituents which would not prevent the claimed compositions orequivalent compositions from producing the expected effects and premixescomprising all or part of the components necessary to the preparation ofthe concrete matrix.

TABLE I Features of the studied organic fibres Mechanical BreakingModule E strength elongation Diameter Length Melting Density (Gpa) (Mpa)(%) (μm) (mm) point (° C.) APV 1.3 20-30  800-1200  7-10 220 RM182 15  6RF350 200 12 RF1500 400 20/30 RF4000 600 30 PEHD 0.97  90 2700 3-5 15050  5 50 10 50 18 PA 1.12 2-5 500-900 20-30 260 500 20 500 30 Steel 7.6210 2900 200 13 1550

TABLE II Evaluation of the mechanical performances of fibre concretemixes: comparison APV fibres/steel fibres Example no. 1 2 3 4 5 6 7 8Portland cement 1 1 1 1 1 1 1 1 Sand 1.430 1.430 1.430 1.430 1.430 1.4301.29 1.29 Quartz flour 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300Silica smoke 0.325 0.325 0.325 0.325 0.325 0.325 0.325 0.325 Adjuvant(dry extract) 0.012 0.012 0.012 0.012 0.012 0.012 0.015 0.015 Water0.200 0.200 0.24 0.24 0.20 0.20 0.24 0.24 Fibres: type Without WithoutSteel Steel APV APV APV APV Fibres: % vol. 2% 2% 4% 4% 4% 4%Wollastonite 0 0 0 0 0 0 10 10 Mica 0 0 0 0 0 0 0 0 Maturing or thermaltreatment 20 90 20 90 20 90 20 90 (T in ° C.) 4 point bending strength16.6 16.5 18.5 21.3 23 27.5 22 27 Tensile strength (Mpa) 7.1 6.75 7.7510.75 9 8.5 9.25 9.45 Tenacity Gc (J/m²) 9.2 10 10 10.5 9.75 10.25 20 22Compression strength 190 198 155 182.5 138 147.5 143 147 Dissipatedenergy Wf (J/m²) 12 15 >5000 >5000 >5000 >5000 >5000 >5000 Fibre type:APV = polyvinyl alcohol

TABLE III Evaluation of the mechanical performances obtained withdifferent organic fibres Example no. 9 10 11 12 13 Portland cement 1 1 11 1 Sand 1.430 1.430 1.430 1.430 1.430 Quartz flour 0.300 0.300 0.3000.300 0.300 Silica smoke 0.325 0.325 0.325 0.325 0.325 Adjuvant (dry0.01 0.01 0.01 0.01 0.01 extract) Water 0.25 0.25 0.25 0.25 0.25 Fibres:type APV 1500 APV 4000 APV 1800 PEHD PAN Fibres: % vol. 4 4 1 1.5Maturing or 90 90 90 90 90 thermal treatment (T in ° C.) 3 point bending35 27 25 22.5 20 strength (Mpa) Tenacity Gc 10 10 11.25 11 10 (J/m²)Compression 142 145 152.5 145 strength (Mpa) APV fibre type = KURARAY(1500, 4000), UNITIKA (1800) polyvinyl alcohol PEHD = high densitypolyethylene PAN = polyacrylonitrile

TABLE IV APV (1500) fibres: effect of the fibre length Example no. 14 1516 17 Portland cement 1 1 1 1 Sand 1.430 1.430 1.430 1.430 Quartz flour0.300 0.300 0.300 0.300 Silica smoke 0.325 0.325 0.325 0.325 Adjuvant(dry extract) 0.01 0.01 0.01 0.01 Water 0.25 0.25 0.25 0.25 Fibres:length (mm) 30 18 12 6 Fibres: % vol. 4 4 4 4 Maturing or thermal treat-90 90 90 90 ment (T in ° C.) 3 point bending strength 35 30 16.5 12(MPa) Compression strength (MPa) 145 152 132.5 135 Breaking type DuctileDuctile No cold Semi- working breakable

TABLE V Example no. 18 19 20 21 22 23 24 25 Portland cement 1 1 1 1 1 11 1 Silica smoke 0.325 0.325 0.325 0.325 0.325 0.325 0.325 0.325 Quartzflour 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Sand 1.43 1.43 1.43 1.43 1.43 1.431.2 1.43 Wollastonite — — — — — — 0.24 — Adjuvant (dry extract) 0.0180.018 0.018 0.018 0.019 0.017 0.018 0.018 Water (E/C) 0.25 0.25 0.220.22 0.19 0.21 0.21 0.25 Fibres: type PEHD PEHD PA PA APV APV APV APVSteel Fibres: length (mm) 5 10 20 30 12 20 20 6 13 Fibres: diameter (mm)0.05 0.05 0.5 0.5 0.2 0.4 0.4 0.015 0.2 Fibres: % vol. 1.5 1 5 5 4 4 4 12 Thermal treatment (T in ° C.) 90 90 90 90 90 90 90 90 3 point bendingstrength 26.5 24.6 20.4 21.3 20.5 24.9 27.9 44.0 40 × 40 × 160 mmCompression strength MPa 121 185 122 139 150 137 140 178

TABLE VI Composition of the matrix used for the pull-out tests Portlandcement 1 Sand 1.43 Quartz flour 0.3 Silica smoke 0.325 Adjuvant (dryextract) 0.018 E/C 0.25 Maturing or thermal treatment (T in ° C.) 90

What is claimed is:
 1. A concrete comprising a hardened cement matrix inwhich organic fibres are dispersed, obtained by blending with water acomposition comprising besides organic fibres: (a) cement, (b) granularelements, (c) fine elements with a pozzolanic reaction, (d) at least onedispersing agent, and wherein: (1) the granular elements (b) have amaximum grain size D of 2 mm at the most, (2) the fine elements with apozzolanic reaction (c) have a particle size of 20 μm at the most, andwherein: (3) the weight percentage of water to the added weight ofcement (a) and the fine pozzolanic reaction elements (c) is in the rangebetween 8% and 25%, (4) the organic fibres have an individual length Iof at least 2 mm and a I/Φ ratio of at least 20, Φ being the fibrediameter, (5) the fibre amount is such that their volume represents 8%at the most of the concrete volume after setting, (6) the ratio Rbetween the average fibre length L and the maximum grain size D of thegranular elements is at least
 5. 2. A concrete comprising a hardenedcement matrix in which organic fibres are dispersed, obtained byblending with water a composition comprising besides organic fibres: (a)cement, (b) granular elements, (c) fine elements with a pozzolanicreaction, (d) at least one dispersing agent, and wherein: (1) the fineelements with a pozzolanic reaction have a particle size of 1 μm at themost, and also wherein: (2) the weight percentage of water to the weightof cement (a) and the fine elements with a pozzolanic reaction (c) is inthe range between 8% and 24%, (3) the organic fibres have an individuallength I of at least 2 mm and a I/Φ ratio, Φ being the fibre diameter,(4) the fibre amount is such that their volume is 8% at the most of theconcrete after setting, (5) the whole elements (a), (b) and (c) have agrain size D75 of 2 mm at the most and a grain size D50 of 150 μm at themost and (6) the ratio R between the average fibre length L and thegrain size D75 of all the elements (a), (b) and (c) is at least
 5. 3.The concrete according to claim 1, wherein the concrete has, in directtensile stress, a ductility, given in terms of ductility coefficient δ,δ>1.
 4. The concrete according to claim 1, wherein the organic fibresare selected from the group consisting of polyvinyl alcohol fibres,polyacrylonitrile fibres, polyethylene fibres, polyethylene fibres,polyamide fibres, polyimide fibres, polypropylene fibres, aramid fibres,carbon fibres and combinations thereof.
 5. The concrete according toclaim 1, wherein the IΦ ratio of the fibres is at the most
 500. 6. Theconcrete according to claim 1, wherein the average adherence stress ofthe fibres in the hardened cement matrix is at least 2 MPa.
 7. Theconcrete according to claim 1, wherein the cement matrix furthercomprises at least one compound, the function of which is to increasethe fibre adherence in the matrix, the compound selected from the groupconsisting of silica compounds, precipitated calcium carbonate,polyvinyl alcohol in aqueous solution, phosphates, latexes, a defoamingagent and combinations thereof.
 8. The concrete according to claim 7,wherein the silica compound is a precipitated silica introduced in acontent ranging from 0.1% to 5% in weight, based in dry conditions, withrespect to the total concrete weight.
 9. The concrete according to claim8, wherein the precipitated silica is introduced into the composition asan aqueous suspension.
 10. The concrete according to claim 1, wherein aportion of the organic fibres is substituted for by metal fibres, themetal fibres having an individual length I of at least 2 mm and a I/Φelongation ratio of at least 20, φ being the fibre diameter.
 11. Theconcrete according to claim 10, wherein the concrete comprises acombination of organic fibres and metal fibres.
 12. The concreteaccording to claim 1, wherein the concrete further comprises elementsthat improve the matrix tenacity, which are either needle-shapedelements or platelet-shaped elements, the average size thereof being 1mm at the most and being provided in a volume proportion between 2.5%and 35% of the added volume of the granular elements (b) and thepozzolanic reaction elements (c).
 13. The concrete according to claim12, wherein the cement has a matrix tenacity of at least 15 J/m². 14.The concrete according to claim 1, wherein the concrete has a matrixtenacity obtained through addition to the cement composition ofreinforcing agents with an anisotropic shape and an average size of 500μm at the most.
 15. The concrete according to claim 14, wherein thereinforcing agents are provided in a volume proportion ranging from 5%to 25% of the added volume of the granular elements (b) and thepozzolanic reaction elements (c).
 16. The concrete according to claim12, wherein the needle-shaped reinforcing agents are selected from thegroup consisting of wollastonite fibres, bauxite fibres, mullite fibres,potassium titanate fibres, silicon carbide fibres, cellulose fibres ortheir derivatives, carbon fibres, calcium phosphate fibres, calciumcarbonate fibres, glass fibres or their derivatives, polyvinyl alcoholfibres, polyacrylonitrile fibres, polyethylene fibres, polyamide fibres,aramid fibres, polypropylene fibres, steel wool and combinationsthereof.
 17. The concrete according to claim 12, wherein the plateletsare selected from the group consisting of mica platelets, talcplatelets, composite silicate platelets, vermiculite platelets, aluminaplatelets, aluminate platelets and combinations thereof.
 18. Theconcrete according to claim 14, wherein at least a portion of theanisotropic reinforcing agents have on the surface a polymer organiccoating selected from the group consisting of polyvinyl alcohol,silanes, siliconates, siloxane resins, polyorganosiloxanes and areaction product between (i) at least a carboxylic acid containing 3 to22 carbon atoms, (ii) at least a polyfunctional aromatic or aliphaticamine or a substituted amine, containing 2 to 25 carbon atoms and (iii)a cross-linking agent which is a hydrosoluble metal complex, containingat least a metal selected from the group consisting of zinc, aluminum,titanium, copper, chromium, iron, zirconium and lead.
 19. The concreteaccording to claim 1, wherein the size of the granular elements (b) isat the most 500 μm.
 20. The concrete according to claim 1, wherein thegranular elements (b) are fine sands or blends of fine sands and eithersieved or crushed sands.
 21. The concrete according to claim 1, whereinsaid granular elements (b) are provided in the range of 20% to 60% inweight of the cement matrix.
 22. The concrete according to claim 1,wherein the pozzolanic reaction fine elements (c) comprise elementsselected from the group consisting of silica, fly ash, blast furnaceslag and combinations thereof.
 23. The concrete according to claim 1,wherein the water percentage based on the added weight of cement (a) andpozzolanic reaction elements (c) is in the range between 13% and 25%.24. The concrete according to claim 1, wherein the concrete has a directtensile stress strength of at least 6 MPa.
 25. The concrete according toclaim 1, wherein the concrete has a 4 point bending resistance of atleast 20 MPa.
 26. The concrete according to claim 1, wherein theconcrete has a compression resistance of at least 120 MPa.
 27. Theconcrete according to claim 1, wherein the concrete has a breakingenergy of at least 2000 J/m².
 28. The concrete according to claim 1,wherein the concrete, after setting, is subjected to a maturing at atemperature close to ambient temperature.
 29. The concrete according toclaim 1, wherein the concrete, after setting, is subjected to a thermaltreatment between 60° C. and 100° C.
 30. The concrete according to claim29, wherein the duration of the thermal treatment is from 6 hours to 4days.
 31. The concrete according to claim 1, wherein the concrete ispre-stressed in pre-tension.
 32. The concrete according to claim 1,wherein the concrete is pre-stressed in post-tension.
 33. A premixcomprising the required components for the preparation of the concreteaccording to claim 1.